Impairment of the mobility of an eye or a permanent different position of the optical axes of the pair of eyes, generally known as being cross-eyed, cause double vision because of the different image segments seen by the two eyes. The superimposition of the two different images in the brain leads to considerable disorientation in the affected person. Therefore, a competition between visual perceptions occurs in the brain of the affected person. The image which receives most of the attention dominates in the long run while the other image is suppressed. As a rule, the dominating image is the image segment which is perceived by the eye having an optical axis positioned normally.
The suppression of the image perceived by the crossing eye leads, in most cases, to vision dimness. This dimness, which is called amblyopia, expresses itself mainly with permanently cross-eyed persons in an inability to see stereoscopically and in an impairment of the faculty to separate different objects, such as letters, within the perceived image.
Especially for children, to improve the dimness of vision in one eye caused by being cross-eyed, the eye with the stronger visual capability is covered. However, this treatment is disadvantageous because the covered eye becomes highly amblyopic within a short period of time. The person treated in this manner will appear, after removing the cover, to be almost blind in the previously covered eye. This phenomenon can often be eliminated by covering the other eye. However, in the cases where correction is not carried out, dimness of vision may remain in the otherwise healthy eye. Being cross-eyed and the resulting vision dimness or dimness from the therapy affects one's ability to self-orientate because of impairment in the field of vision.
Devices to ascertain the angle of crossing are known, for example the work of Josef Lang: Strabismus, published by Hans Huber, Stuttgart, 1986. The angle of crossing is the deviation in the position of the optical axes of both eyes. For the purpose of measuring the angle of crossing prisms, and other devices, are used. These prisms and other devices can be used to determine the angular deviation by covering one or the other eye and by observing the eye reaction. However, this process is subject to the risk of faulty diagnostic results due to faulty operation.
Furthermore, another device called a synoptophore can be used for diagnostic purposes. The synoptophore device is equipped with two small pipes swivelling on an angle scale. One end of each pipe is placed before the eyes of the person to be examined while the other end is directed upon two different objects. By swivelling at least one of the pipes from a starting position representing normal vision, the objects are portrayed in front of the eyes of the examined person so that he/she finally receives the impression of a registering, or stereoscopic image. The swivelling angle of the pipe in relation to the starting position reveals the angle by which the eyes cross in the examined person.
In addition to this process, a method is known to find the crossing angle using a device called a "video eye tracker". The video eye tracker is designed for the observation and monitoring of eye movement in order to determine the smallest angle by which eyes cross. The video eye tracking system comprises an infra-red camera the image of which may be evaluated by a computer, for example.
These known devices for determining the crossing angle are suited only for one-time examination under conditions which exist only in a doctor's office.
Therefore, it has been tried to afford cross-eyed persons a stereoscopic vision by means of prism glasses. The prism assigned to the crossed eye is set to take into account the angle by which it is crossed when looking straight ahead.